Hacking is judged on your horse's appearance, but also on how well he is going. If your horse looks beautiful, but is not well trained and has nice paces he won't do well in the hack ring.
Before entering the ring make sure your horse and your clothing are absolutely spotless. You should have already applied face make up, quarter marks (if you're using them) and painted his hooves. Make sure your hair is very neat and your hunt cap on straight. Check that all straps are in their keepers and that everything is correctly organized. Remove any stray hairs from your numnah.
When your class is called enter the arena walking in an anti clockwise direction. Judging doesn't begin until the judge says 'walk on please', so make sure that you're in a good position and not being crowded or are stuck behind anyone. When the judge asks everyone to walk, make sure your horse is correctly flexed on the bit and is walking forward freely. The judge will then ask all competitors to trot, and then canter. She may ask the class to change direction, she will do this by asking one rider to cross across the ring, and then everyone else will follow. She will then start calling horses in, either by calling your number (make sure you know it!) or by saying something like 'that bay with the stripe' or something similar. If you are called in line up to the left of whoever was called in before you, and don't let your horse rest a leg. When enough horses are called in (anything from four to the whole class) the judge will say 'Thankyou', or 'that's all thankyou' and if you haven't been called in you then leave the arena at a walk.
The judge will now describe the workout to you. Repeat it back to the judge if you aren't sure about it. The rider who was called in first will perform the workout first, and then the person who was called in second and so on. It is during the workout that you have a chance to really show off your horse, so make sure you listen carefully and don't make the workout to small or too large (both common mistakes as the workout is normally done in an unenclosed area). When all competitors have completed their workout the judge will normally stand before you and pointing to each place getter, will say 'the bay first, that chestnut second, that bay third' or similar. If it is a rider class the ribbon is tied around your left arm, if is is a hack class it is tied around your horse's neck. When given your ribbon say, 'Thankyou' or 'Thankyou judge', and after all ribbons are awarded turn to the rider on either side of you and congratulate them. Some riders will say something like 'Thankyou judge, congratulations riders' after the ribbons are handed out. While the judge is awarding you your ribbon they may give you advice on what you did wrong, or right. This can be really helpful!
Leave the arena with the winner leaving first, then second and third, and the non place getters leaving last.